Because lead poisoning results from ingested lead pellets, the occurrence of lead shot in waterfowl gizzards provides information on the degree of exposure. Although die-offs show that waterfowl continue to ingest shot after the hunting season, large numbers of waterfowl gizzards cannot be obtained easily except during the hunting season. Until recently, therefore, much of what was known about shot ingestion was derived from the examination of waterfowl gizzards collected from hunters during the fall and early winter. Bellrose (1951) found that 3-4 percent of the ducks livetrapped and examined by fluoroscopy before the hunting season had shot in their gizzards. The numbers increased steadily until December when 12 percent of the gizzards contained lead.
Bellrose (1959:262-263) analyzed 39,610 gizzards collected from waterfowl in habitats scattered across the United States and in British Columbia. An average of 6.6 percent of those gizzards contained one or more ingested shot. His findings also showed that lead pellets occurred in gizzards from waterfowl in all regions, although the incidence of ingested lead varied by region and species. Such generalized occurrence is understandable because the wetlands bring waterfowl and hunters together. Because wetlands are so widely distributed, the incidence of ingested lead is not confined to focal points of infection as is the case in diseases caused by pathogens.
Subsequent studies indicate that the shot ingestion rates reported by Bellrose in 1959 were low. Montalbano and Hines (1978) evaluated three techniques for determining the presence of shot in waterfowl gizzards: manual examination, X-rays of intact gizzards, and X-rays of gizzard contents. They obtained contents of 300 lead-free gizzards, seeded them at varying rates with varying numbers of sanded and partially eroded shot removed from other gizzards, and asked 3 cooperators to examine the gizzards manually for lead shot and then to examine X-rays of the gizzard contents. They found that manual examination missed 24 percent of the ingested shot and X-rays of intact gizzards missed 28 percent. Manual examination missed small, eroded pellets camouflaged among seeds and grit; X-rays of intact gizzards sometimes failed to detect small shot obscured by grit, food, and the muscular wall of the gizzard. Sporre and Blevins (1981: 19) compared manual and X-ray examinations of 998 waterfowl gizzards in Indiana. Manually, they found ingested shot in 6.7 percent of the gizzards; by X-ray they found shot in 10.4 percent. In an Illinois study, Anderson and Havera (1985: 29) concluded that manual examination underestimates ingestion of shot pellets by 20-25 percent.
A compilation of data from studies that reported shot ingestion rates from 1973 through 1984 shows that 8.9 percent of 171,697 duck gizzards analyzed contained shot pellets (Table 1). Because these data are more recent and from a larger sample than data obtained earlier, we employ them in subsequent calculations. Because lead-poisoned birds at all levels of ingested shot are about 1.65 times more likely to be bagged by hunters than healthy ducks (Bellrose 1959: Table 20), we calculate that the population actually contained closer to 5.4 percent lead-poisoned birds at any one time during the fall - a reasonable reduction from 8.9 percent because of the inflated bag rate of poisoned ducks. However, a much larger percentage of all ducks actually consume lead shot in any given year - perhaps as much as one-third of the continental population. This estimate is reached as follows:
Because of varying habitats and feeding habits, waterfowl species differ greatly in their rate of shot ingestion. From 1938 through 1953, the bay diving ducks - canvasback (Aythya valisineria), lesser scaup (Aythya affinis), redhead (Aythya americana), and ring-necked duck (Aythya collards) - had the highest rate of shot ingestion among waterfowl (12- 14 percent of the gizzards analyzed); mallard, black duck (Anas rubripes), and pintail (Anas acuta) had an intermediate rate of shot ingestion (7-9 percent); green-winged teal (Anas crecca carolinensis), shoveler (Anas clypeata), wood duck, gadwall (Anas strepera), blue-winged teal, and wigeon (Anas americana) had the lowest rates of shot ingestion (1-3 percent) (Fig. 1).
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| Figure 1 - Percentage of gizzards with varying numbers of ingested lead shot in 35,220 gizzards of various duck species collected at many locations in North America, 1938-1953. Data from Bellrose (1959:260). |
The percentage of waterfowl that ingest shot depends upon hunting pressure and other variables. Because these factors vary from region to region and from year to year, so does the percentage. Bellrose (1959: Table 12) found that from 1938 through 1953 a relatively small proportion of ducks from the Dakotas, Nebraska, Colorado, and Missouri ingested shot (2-3 percent of the sample); however, an exceptionally high percentage of ducks from Indiana, Louisiana, Texas, and British Columbia ingested shot (11-21 percent). Between 5 and 9 percent of ducks from other states ingested shot. The relative proportions of mallards and black ducks and all other ducks ingesting shot in the four flyways are given in Figure 2.
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| Figure 2 - Regional comparison by flyway of the percentage of mallards and black ducks ingesting lead versus percentage of other ducks, 1938-1953. Data from Bellrose (1959:262-263). |
Since Bellrose's studies on the rate of shot ingestion, numerous studies during the 1970s and early 1980s have provided much more information on this subject (Table 1), (Table 2), (Table 3). Higher percentages of most species of ducks and all species of geese ingested shot during 1973-84 (Table 1) and (Table 3) than during 1938-1953 (Bellrose 1959:260). As was true for Bellrose's study, ducks may readily be categorized according to their propensity to ingest shot (Table 1): the bay diving ducks and the mottled duck (Anas fulvigula) have the highest propensity (12 to 28 percent of the sample); mallard, black duck, and pintail have moderate rates (8 to 12 percent); and wood duck, teals, wigeon, gadwall, and shoveler have the lowest rates (1 to 3 percent). Shot ingestion among Canada geese (Branta canadensis) varied by state from 1.9 to 44.1 percent; variation among snow geese (Anser caerulescens) was from 0.0 to 4.2 percent (two states). The rate was 4.3 percent among 494 white-fronted geese (Anser albifrons frontalis) from Texas (Table 3).
Table 3 - The rate of shot ingestion (lead and steel) by geese in various states of the four flyways.
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aNumber of gizzards examined.
bPercentage of gizzards with >/=1 shot.
cThere were 39 tundra swans examined from Nevada and 5.1
percent contained ingested shot.
With few exceptions, states with extensive wintering populations of waterfowl had the highest rates: Florida, Louisiana, and Texas. Waterfowl in Oregon showed an excessive rate, but all the data were from Sauvie Island, an area noted for high rates of lead shot ingestion due to hard bottoms. Other areas where shot ingestion was exceptionally high were Merritt Island, Florida; Catahoula Lake, Louisiana; and Murphree Wildlife Area, Texas.
Although some localities within the broad confines of the flyways have low rates of shot ingestion, others have extremely high rates. Except for the Great Plains region of the United States, appreciable numbers of waterfowl ingest shot. Mudge (1983: 340) reported the incidence of shot ingestion among 2,453 British wildfowl of 18 species (ducks, geese, swans, and coots) shot, 1979-81, and in 230 wildfowl of 21 species (ducks, geese, and swans) found dead during the same period. The percentage of shot birds with ingested lead pellets ranged from zero for nine species to 4.2 for mallards, 6.7 for goldeneye (Bucephala clangula), 7.1 for greylag goose (Anser anser), 10.9 for pochard (Aythya ferina), 11.7 for tufted duck (Aytha fuligula), and 11.8 for gadwall. Of 35 mallards found dead, 17.1 percent had lead pellets in their gizzards. In a comparison of regional variation, Mudge (p.360) found that 6.0 percent of mallards shot at inland sites had ingested pellets; only 2.6 percent of those shot on coastal areas had ingested pellets.
Most investigators agree that spent shot from the guns of hunters is the main source of lead that poisons wild waterfowl. Bellrose (1959) reported that the number of spent lead pellets found on 24 areas in 7 states and provinces ranged from 0 to 291,579 per ha, an average of 69,847 per ha (0 to 118,083 per acre, an average of 28,277 per acre). Mudge (1984:299) reported densities of lead shot in the top 15 cm of wetland soils at 22 sites in 9 areas of Britain. No pellets were found at 3 sites, but at the others densities varied from 2.04 pellets per m2 to 30.0 per m2 (20,388 to 300,000 pellets per ha).
In addition to the prevalence of lead shot, several other factors influence the ingestion of shot by waterfowl. These include feeding habits, firmness of the lake or marsh bottom, depth of water, size of pellets, ice cover, and season (Bellrose 1959). In spring, high water often decreases the availability of lead pellets to ducks. Little carry-over of lead shot available to ducks from one season to the next has been found in feeding areas with soft bottoms.
The Winchester Group (1974) argued that lead in the wing bones of ducks as reported by the U.S. Fish and Wildlife Service (1974) could come from low-level exposure to environmental lead and not from ingested lead pellets. Because scientists from the U.S. Fish and Wildlife Service had analyzed the bones of immature mallards that had had only a few months in which to accumulate lead, they assumed that when two distributions of lead were found, the higher level represented ingested lead shot and the lower other environmental sources of lead. Lead levels in wing bones from mallards in the Mississippi, Central, and Pacific flyways fitted this assumption, as did levels in birds from Virginia and North Carolina in the Atlantic Flyway. Bones of birds from Massachusetts, Maryland, New Jersey, and Georgia, however, fitted one simple exponential probability distribution. Lead from sources other than shot could have contributed appreciable amounts of lead to the wing bones of mallards from these four states, but no source other than lead pellets was identified to explain the high levels found. Thus, the U.S. Fish and Wildlife Service (1976) concluded that lead pellets are the primary source of lead available to wild waterfowl.
Niethammer et al. (1985) found low levels of lead (geometric means of 0.07-0.55 ppm, wet weight) in the livers of green-backed herons (Butorides striatus) collected downriver from lead mines in Missouri. They found somewhat higher levels (0.23-2.39 ppm, wet weight) of lead in the carcasses of northern roughwinged swallows (Riparia riparia) from a colony nesting in a mine tailings pile containing lead concentrations from 2,360 to 26,600 ppm dry weight. Presumably the respiratory and dermal exposure to lead-contaminated dust explained the higher lead levels in the swallows. Niethammer and his colleagues did not report clinical symptoms of lead poisoning in either the herons or the swallows.
Additional examples of birds that have acquired lead from sources other than shot include laughing gulls (Larus atricilla) collected near Galveston, Texas (Munoz and Gesell 1976). Adults and prefledglings had lead concentrations ranging from 0 to 16 u/g of wet weight in liver, brain, heart, and skeletal tissues. No significant difference between gulls in the nest and adults was found, an observation that suggests that the young birds had reached an equilibrium regarding lead before they left the nest. No lead toxicosis was reported in these birds. Siegfried et al. (1972) found a significant difference in the mean content of lead in laughing doves (Streptopelia senegalensis) collected in the city of Cape Town, South Africa, and those collected in a rural area 50 km away. They concluded that the difference resulted from the higher level of lead in the city atmosphere. Ohi et al. (1974) found a significant difference in the lead content of femurs from pigeons collected from a farm house in a rural area and those collected from a crowded temple in downtown Tokyo. Higher levels in the city birds were attributed to atmospheric lead from automobile exhaust. No mention was made of clinical symptoms or mortality from lead poisoning in either the Cape Town or the Tokyo study.
Several reports of zoo animals suffering from lead poisoning, which in some cases was fatal, are available. Bazell (1971: 130), for example, studied animals in the Staten Island Zoo and observed that although "some of the lead in the animals' bodies may have come from paint in their cages, the major source appears to be atmospheric contamination." Unlike these zoo animals, wild waterfowl are seldom exposed to high levels of atmospheric lead for extended periods.
We are concerned about the levels of lead pollution in the atmosphere, soils, water, and plants of the world; however, we found no evidence of extensive mortality from lead poisoning in wild animals other than lead poisoning in waterfowl as a result of ingesting lead pellets.